A conventional subsea wellhead assembly includes a wellhead housing that supports one or more casing hangers, each located at the upper end of the string of casing extending into the well. A production tree landed on the wellhead housing controls the production of well fluids. A tubing hanger supports a string of tubing through which the well fluid flows. The tubing hanger may be located either in the wellhead housing or in the production tree. The tree has a choke and valves to control the flow. The tree may also have sensors for monitoring pressure, temperature and flow rate.
The more sensitive components of the tree are the flow interface devices, such as the choke, flow meter, and pressure and temperature sensors. U.S. Pat. No. 6,460,621 discloses a modular tree that has a lower module containing the valves. The upper module contains the more sensitive equipment and lands on the lower module. The upper module can be retrieved separately from the lower module for repair or replacing the flow interface devices
In the '621 patent, the tree block for the lower module has two vertical passages, one for the production flow and one for communication with the tubing annulus. The upper module has also two vertical passages, one for upward flowing fluid from the production passage of the lower module, and the other for flowing fluid downward back into an upper section of the tubing annulus passage in the lower tree block. A port in the lower module connects this upper section of the tubing annulus passage to a flowline connector. A valve selectively blocks the upper section of the tubing annulus passage from the lower section while the upper section is serving as a production flow passage.
While the design of the '621 patent is feasible for many applications, the side-by-side vertical production and tubing annulus through-bores restrict the diameter of the production passage. In some instances, very large production passages are desired for wells, particularly for high flow rate gas wells.